1. Field of the Invention
The present invention relates to materials science and more particularly, to the application of phosphor to the fabrication of a warm white luminous semiconductor. This luminous semiconductor uses a spectrum conversion layer prepared from a light-permeable polymeric material and an inorganic phosphor powder distributed therein. The phosphor is excited by a short wave radiation, for example, violet or blue light and a long wave radiation, for example, yellow-orange light, and combined with a first order short wave to produce white light of which the optical parameter, for example, color temperature relies upon the spectrum parameter of the phosphor.
2. Description of the Related Art
More than 10 years ago, researchers at Nichia Chemical Industries in Japan announced the development of a continuously operating laser diode that emits light at blue wavelengths (see S. Nakamura et. al and Blue laser, Sringer Verlag, Berlin 1997). Based on this research, new blue laser architectures have been developed for many commercial applications. Before Nakamura et. al, many effective luminous semiconductors (light emitting diodes) researches were known (see V. Abramov et. USSR. 1977). InGaN heterostructure-based blue LED becomes one of important semiconductor luminous elements. During 1998˜2000, many effective white LEDs were developed (see Schimisu et. al's U.S. Pat. No. 5,988,925 Jan. 7, 1999 and E. Ellens et. al's U.S. Pat. No. 6,670,714 Dec. 30, 2003). These architectures employ two basic concepts. One concept is the technique of combining two compensation colors, i.e. blue and yellow, subject to the Newton's law of complementary colors, to create white radiator. This technique was intensively applied to create CRT screen for black-and-white TV. The other concept is the use of semiconductor nitride heterostructure with yellow phosphor. The heterostructure radiates blue light. The phosphor is excited by wide band light wave. When putting the inorganic phosphor into optical contact with the blue heterostructure, it absorbs a certain fraction of the blue light radiation from the blue heterostructure and emits light at yellow wavelengths. At this time, the blue light that is not absorbed by the phosphor is combined with the yellow radiation, producing white light.
These white semiconductor light sources have some particular properties, such as: 1. When half angle 2θ=10˜120°, the radiation intensity is as high as several tens or several hundreds of candelas; 2. White radiation has high lumen flux, 1 several tens of lumens on the area of one heterostructure; 3. The specific color temperature of the semiconductor luminous radiator can be adjusted from T=12000K through T=4000˜5000K.
According to the characteristics of radiation spectra, white LEDs, based on blue light and yellow light, have two spectrum maximum values. These white LEDs are called two-dimensional light emitting compositions (see N. Soschin. “LED and lasers”, N1-2, 2002). More 10 years ago, the phosphor based on inorganic powder type YAG (yttrium aluminum garnet) substrate, excited by cerium, and had stoichiometric equation (Y,Gd,Ce)3Al5O12. This phosphor was intensively used in electronic radiographic apparatus (see C. Schionoya et “Handbook of Phosphors”. CBC precc NY, 1999.). For LED application, Gd ion is added to make a modification.
In (Y,Gd,Ce)3Al5O12, modifying the spectrum composition of Ce+3 cause shift of the position of the spectrum maximum value λ=528˜562 nm. When the content of Gd ion reaches 0.2 atomic fraction, the radiation spectrum energy is shifted to λ=568 nm. However, the aforesaid phosphor cannot achieve maximum radiation shift λ>569 nm. In 2005 (see N. Soschin et. al and US 2005 0088077 A1 patent application), the applicant of the present inventor studied the material of (Y,Gd,Ce)3Al5O12:Pr. By means of adding Pr+3, the material radiates at λ=610 nm. The applicant of the present invention uses this patent as a reference. Although this material can produce orange light, it still has a substantial drawback, i.e., low radiation quantum efficiency. Further, in known phosphors, the fraction of red-orange light is low.
In 2006, the applicant of the present invention studied the chemical formula of a phosphor composition having garnet crystal architecture. When compared with the chemical formula of conventional synthetic Ln3Al5O12 garnet, natural Me+23Me+32Si3O12 garnet show similar applicability. Based on this garnet architecture, the unit lattice contains the atomic number of z=20 atoms. These atoms have respective coordinates. With respect to the three Me+2, the coordination number is K=8. At this time, oxygen ion forms the initial range of coordination, wherein Me+2=Mg+2, Sr+2, Ca+2, or Ba+2 in few cases. Me+3 includes VIIIB group +3 elements, such as Fe+3, or rare earth +3 ion Me+3=Ln+3═Y+3, Gd+3, Lu+3. Normally, the coordinate range of these ions is eight O−2. In the gap between big size ions Me+2 and Me+3, small size IVA group element ions exist, such as Si+4, Ge+4, Sn+4. These ions have a small radius, and can be coordinated by a small number of oxygen ions, usually, KSi=4.
Since 2005, the applicant of the present invention has synthesized many luminous materials having Me+23Ln2Si3O12 garnet natural stoichiometric equation. In this natural architecture, there are two lattice nodes. These lattice nodes allow allocation of activator ions, and have different degrees of oxidation, Ak+2 and Ak+3. The applicant of the present invention identified these activator ion pairs, such as Eu+2 and Ce+3; Eu+3 and Pr+3; Sm+2 and Pr+3; Eu+3 and Dy+3. The properties of these luminous compounds were described in a report issued by the present invention in 2007 (see N. Soschin V Conf. of AIIIBV Moscow, 2007, h.) (AIIIBV Moscow, January, 2007).
Specialists from “General Electric” (see F. Srivastava et and US pat 2006 284196 Dec. 21, 2006.) invented patented phosphor composition (Mg, Ca)3Ln2SiO12, excited by Ce+3. This phosphor has the advantages of: 1. orange-red luminous spectrum maximum value λ=620-640 nm; 2. high absorption of first order blue radiation of semiconductor heterostructure; 3. low temperature preparation method.
The aforesaid silicate-garnet phosphor is not widely used for different applications due to certain drawbacks. The first drawback is its high luminous spectrum half-wave width, λ0.5≧=115 nm and low radiation lumen equivalent value QL. Because λ=640 nm radiation spectrum shifts toward λ>720 nm red wavelengths. This region is not sensitive to human eyes. The total radiation lumen equivalent value of this phosphor does not surpass QL=180˜200 lm/W. This value is lower than the radiation lumen equivalent value of (Y,Gd,Ce)3Al5O12 synthesized garnet QL≈290˜360 lm/W.
The second drawback is that, having a LED produce white or warm white radiation requires a radiation at the spectrum region λ=620˜640 nm, therefore, when comparing the proposed phosphor with standard garnet, a big fraction (over 50%) is required.
The third drawback is that due to a great size difference between the substrate ion Ln+3(DLn=0.86 A) and the activator ion Ce+3 (DCe=1.12 A) and the concentration limitation of Ce+3 in the lattice [Ce+3]≦0.01 atomic fraction, the phosphor has a low radiation quantum efficiency, not suitable for creating a warm white LED having high radiation intensity.